The invention relates to a premix burner arrangement with catalytic combustion for operating a combustion chamber of a gas turbine arrangement, as well as a method to this effect, having a premix burner, wherein at least one fuel addition unit as well as inlet air openings have been provided in such a way that gaseous and/or liquid fuel can be mixed with combustion inlet air inside the premix burner and form a fuel/air mixture, which exits from the premix burner downstream in the direction towards the combustion chamber positioned after the premix burner arrangement and which can be ignited inside the combustion chamber.
A premix burner arrangement of this type is disclosed in EP 0 833 105 A2. The premix burner described in this document is provided with a central fuel nozzle axially towards the swirl generator, said swirl generator being followed downstream by a mixing section in which the atomized fuel is mixed completely with air to form a fuel/air mixture. At the downstream outlet of the mixing pipe, the combustion chamber is provided, inside which a spatially stable flame zone forms when the premix burner is operated.
Even though the known premix burner arrangement has been designed with a view towards improved emission values, i.e., in particular reduced NOx emission values, by providing a mixing section, an objective is in particular to farther improve the waste gas values of combustion systems of this type. Furthermore, so-called combustion chamber vibrations occur during the operation of known premix burner arrangements, to the great disadvantage of flame stability. Such combustion chamber vibrations or pulsations have a particularly disadvantageous effect in operating states where lean fuel/air mixtures are used. This results in high lean extinction limits, i.e., an extinction of the flame in spite of relatively high fuel content, causing the operating range of the premix burner to be greatly limited, especially with respect to a lean operation.
Previous approaches and attempts to combine premix burner arrangements with catalyzers in order to reduce at least the previously mentioned NOx emissions in the combustion process failed or provided only unsatisfactory results, especially since the usual use of liquid fuel, for example oil, for firing such premix burner arrangements makes catalyzers known per se unusable because it obstructs the catalyzer openings.
The invention is based on the objective of further developing a premix burner arrangement with catalytic combustion for operating a combustion chamber of a gas turbine arrangement according to the preamble of claim 1 in such a way that on the one hand, measures are implemented through which the NOx emission values are supposed to be substantially reduced. And in addition, the measures should result in flame stabilization within the combustion chamber so that the operating ranges of the premix burner arrangement are expanded, in particular with respect to an improved lean extinction limit.
The realization of the objective of the invention is disclosed in claim 1. The subject of claim 9 is a method for operating a premix burner arrangement with catalytic combustion. Characteristics that advantageously further develop the concept of the invention are the subject of the secondary claims and specification in reference to the exemplary embodiments.
According to the invention, a premix burner arrangement according to the preamble of claim 1 is further developed in such a way that, prior to the entrance of the fuel/air mixture into the combustion chamber, a catalyzer unit is provided, through which part of the fuel/air mixture can be introduced and passes through, before this part flows, together with the remaining portion of the fuel/air mixture into the combustion chamber.
An essential aspect in using a catalyzer unit within an actually known premix burner, which is usually operated with liquid fuel such as, for example, oil, is the positioning of the catalyzer unit in an area downstream from the atomizing nozzle of the premix burner in which the fuel/air mixture is completely mixed and the liquid fuel is spatially distributed very finely or is already largely evaporated. If fuel in the previously mentioned form enters a catalyzer, it is possible to convert the fuel at least in part catalytically inside the catalyzer, for example by means of a thermal and/or chemical conversion, without destroying or obstructing the catalyzer material itself in the process.
According to the invention, only part of the fuel/air mixture spreading through the premix burner in the direction towards the combustion chamber passes through the catalyzer unit, especially since the catalyzer unit preferably is positioned concentrically at the outlet of the premix burner or the mixing section, just before the entrance into the combustion chamber, so that only peripheral partial flows of the fuel/air mixture spreading in the direction towards the combustion chamber flow through the catalyzer unit. After the at least in part catalytically converted fuel/air mixture exits from the catalyzer unit, the at least partially converted mass flow reaches edge areas of the flame front that forms inside the combustion chamber, so that the flame itself can be clearly stabilized. The effects of the flame stabilization due to the peripheral mass flows become especially obvious in operating states where lean mixtures are used. An especially important indication of the positive effect of the mass flows entering the combustion chamber peripherally can be seen in the decrease of the lean extinction limit, which makes it possible to significantly expand the operating ranges of the premix burner. Combustion chamber pulsations also occur much less.
Because of the peripheral arrangement of the catalyzer unit in relation to the fuel/air mixture that spreads axially inside the premix burner, an unlimited operation with liquid fuel such as, for example, oil, is possible, in spite of the presence of the catalyzer unit before the combustion chamber. Since the fuel injection takes place centrally in relation to the premix burner axis, but the catalyzer unit is positioned as far as possible from the fuel injection, and only at the outer periphery of the spreading fuel/air mixture, the catalyzer unit is able to withstand an oil combustion inside the burner system without damage.
In principle, it is possible to position the catalyzer unit distributed in a partial or completely circular manner around the mixing pipe, whereby a circularly extending outlet channel inside the mixing pipe is provided for the partial separation of peripheral partial flows of the fuel/air mixture spreading inside the premix burner. The inside diameter of the mixing pipe hereby can remain almost unaffected. In another embodiment, the catalyzer unit is provided on the inside, downstream at the exit of the mixing pipe in such a way that peripheral partial flows of the fuel/air mixture in a forcibly guided manner pass through the catalyzer material, especially since the catalyzer material narrows the flow cross-section within the mixing pipe at the latter""s exit towards the combustion chamber.
Also conceivable are constructions in which the catalyzer unit is constructed between the mixing pipe and combustion chamber in the manner, for example, of a pin diaphragm.